Feedforward amplifiers provide a means of amplifying signals with a minimum of distortion. They are particularly useful in cable communication systems of an extended length where repeated amplifications are necessary to maintain signal quality.
In various cable communication systems, for example cable television networks, signal strength and quality must be maintained over long distances. Such systems maintain the signal at the desired level by use of a series of amplifiers positioned along the cable. Some cable communication systems are currently being designed to carry broad bandwidth signals, e.g. 50 MHz to 550 MHz plus frequencies.
Maintaining signal quality in a long distance, broad bandwidth cable communication system represents a difficult technological challenge. The strength of the original signal will decrease generally in proportion to the length of the cable over which the signal is transmitted, and must be periodically restored by amplifiers distributed along the cable. Distortion may be introduced at the various amplification stages by a wide variety of causes, e.g. inherent bandwidth limitations of the amplifier, variations in the signal carrying properties of the cable, external signals, temperature variations and other environmental effects. Since introduction of appreciable distortion at each amplification stage would result in unacceptable signal quality after repeated amplifications, minimumization of distortion at each amplification stage is critical.
Feedforward amplifiers reduce such distortion by combining the unamplified, incoming signal with the amplified signal, suitably attenuated, to obtain a signal component representative of the distortion, phase inverting and amplifying that component, and then recombining the phase inverted distortion component with the amplified signal. The phase inverted distortion component thus cancels the distortion component in the amplified signal, leaving the amplified signal relatively free of distortion. The operation of a feedforward amplifier of the type considered in the preferred embodiment of the present invention is described in U.S. Pat. No. 4,472,725 for LC Delay Line for Feedforward Amplifier, to Robert M. Blumenkranz.
To obtain the distortion component, the amplified signal (with its inherent distortion) is first attenuated then combined with the original, or incoming, signal, suitably delayed. The delay must be adjusted to achieve complete cancellation of the (attenuated) amplified signal by the incoming signal, thereby allowing complete cancellation of the distortion component of the amplified signal and the phase inverted distortion component. Due to the necessity of exact cancellation of the distortion component and phase inverted distortion component, any change in the phase or relative strengths of the signals introduced by the feedforward circuitry or by external factors can substantially increase the distortion in the output signal. One cause of such a change is a change in the ambient temperature of the feedforward amplifier. Temperature induced changes in gain of the IC amplifiers employed in such feedforward circuitry, and corresponding changes in phase of the amplified signals, are particularly significant in a cable communication systems where wide variations in ambient temperature may be experienced and the IC amplifiers must operate over a very broad bandwidth in a system that cannot tolerate cumulative distortionn effects.
It is an object of the present invention to provide a feedforward amplifier which maintains relatively distortion free amplification over a wide temperature range and broad bandwidth. This and other objects of the present invention will be apparent to those skilled in the art.